Manipulating The Conformation,Morphology And Photoelectronic Properties Of Polydiarylfluorenes By Supramolecular Steric Hindrance Effect

High stability,high crystallinity and high luminscent quantum efficiency are the necessary factors for commercial applications of polyfluorenes.Although the ?-phase thin films with good quantum efficiency can be obtained by controlling the alkyl chain length of polydialkylfluorenes and external conditions,the spectral stability and the morphology stability are not solved.Therefore,an effective strategy to break the commercial bottleneck without decreasing the quantum efficiency while improving the stability and crystallinity of polyfluorenes is imperative.Based on synergistically molecular attractor-repulsor theory(SMART),a kind of polydiarylfluorene with high stability,high crystallinity and high luminous efficiency was obtained by repeated rational molecular design and practical experiments,which was succesfully used in the preparation of highly efficient and stable luminescence device.First,we investigated the effects of different steric groups on the ?-conformation,gelation,crystallinity,morphology and electroluminescence properties of PFO by introducing three different steric hinders.It is found that the introduction of the hindered groups will improve the stability but strongly inhibit the formation of ?-phase.The balance of ?-phase and stability can be achieved in the gel state.On the basis of this,we obtained a kind of polydiarylfluorene(PODPF)crystal film containing ?-phase by 220 oC thermal annealing combining with rational molecular design and external condition control.The films exhibited similar PL spectral behavior(redshift and narrow emission width)and Raman spectral changes(strong signal enhancement at 1303 cm-1 and low frequency movement at 1600 cm-1)with PFO.The crystallization characteristics of the polymer were confirmed by AFM and GIXD.And the double conformational doped films resulted in dual-emission ASE phenomenon.Finally,the ?-phase of the polymer was introduced into the PLED to obtain an efficiency of twice as much as the amorphous phase and better EL stability.In order to further clarify the effect of molecular structure on the properties of PODPF,we demonstrated the molecular weight effect,steric hindrance effect,and alkyl chain length effect,respectively.First of all,we systematically compared three different molecular weights of PODPF and found that high molecular weight PODPF exhibited a stronger chain aggregation due to poor solubility,which was more likely to form ?-phase,as well as increased crystallization and selfassembly ability.Then,the effect of different hindered groups on the ?-phase formation was studied by copolymerizatin of 20%.It was found that the introduction of steric hinders improved the thermal stability of polydiarylfluorenes,but at the same time strongly inhibited the ?-phase and the crystallinity,similar with the effect of steric hindrance on PFO.Finally,Pn DPF system(P6DPF,P7 DPF,P8DPF,and P9DPF)with different alkyl chain length was developed.The results showed that the alkyl chain length of C7 and C8 can form ?-phase.Raman spectroscopy further demonstrated that the degree of planarization was: P8DPF> P7DPF> P6DPF> P9 DPF.AFM and GIXD films showed that the crystallization parameters of the films were related to the twist angle changes caused by the length of Pn DPF alkyl chains.With Pn DPF as the research template,which was characterized with high stability,luminous efficiency and crystallinity,the alkyl chain effect of light-emitting conjugated polymer with steric effect was studied for the first time.Based on the works of the previous chapters,we investigated the growth and properties of selfassembled nanowires by POM,SEM,AFM,TEM and GIXD in combination with high crystallinity,high stability and high luminous efficiency of P6 DPF.And its application in PLED revealed a precedent of study on polymer nanowires PLED based on wide band-gap semiconducting materials,which provided an excellent template for studying the photophysical properties of 1D nanostructures.In conclusion,from the point of view of molecular design,this paper has obtained a kind of high stability,high crystallinity,and high light emission materials by screening the elements such as steric hindrance,alkyl chain length and molecular weight under the guidance of SMART,and stepped forward to the commercialization of polyfluorene materials.